block: bio: pass bvec table to bio_init()

Some drivers often use external bvec table, so introduce
this helper for this case. It is always safe to access the
bio->bi_io_vec in this way for this case.

After converting to this usage, it will becomes a bit easier
to evaluate the remaining direct access to bio->bi_io_vec,
so it can help to prepare for the following multipage bvec
support.

Signed-off-by: Ming Lei <tom.leiming@gmail.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>

Fixed up the new O_DIRECT cases.

Signed-off-by: Jens Axboe <axboe@fb.com>
diff --git a/fs/block_dev.c b/fs/block_dev.c
index b0c790a..7022ddc 100644
--- a/fs/block_dev.c
+++ b/fs/block_dev.c
@@ -222,9 +222,7 @@ __blkdev_direct_IO_simple(struct kiocb *iocb, struct iov_iter *iter,
 			return -ENOMEM;
 	}
 
-	bio_init(&bio);
-	bio.bi_max_vecs = nr_pages;
-	bio.bi_io_vec = vecs;
+	bio_init(&bio, vecs, nr_pages);
 	bio.bi_bdev = bdev;
 	bio.bi_iter.bi_sector = pos >> 9;
 	bio.bi_private = current;